1. Field of the Invention
The present invention relates to generating full color halftone images having smoothly varying grey-scale levels, and more particularly, to conditioning a generated image prior to a halftoning step, so that the halftoning step generates smoothly varying full color halftone images while avoiding the generation of visible halftone texture patterns and other printer-artifact patterns.
2. Description of the Related Art
Full-color electronic printers use a plurality of inks, such as cyan (C), magenta (M), and yellow (J) in three-color printers and C, M, J and black (K) in four-color printers. In such three- and four-color printers, different levels of color saturation for the three non-black colors (CMJ) are generated by using a halftoning process. In addition, the full spectrum of colors is generated by selectively overlapping each of the different separation colors. In a three-color electronic printer, black is generated by overlapping all three of the separation colors, while in a four-color printer, black can be either generated by combining the three separation colors (process black) or by using the black ink (printer black).
In halftoned image generation, the individual pixels, which comprise the image plane upon which the original image is mapped, are grouped into small or unit cells. A halftone screen is applied to the unit cells of the original image to generate a halftone image suitable for printing on an image reproduction apparatus. For example, U.S. Pat. No. 4,868,587 to Loce et al., assigned to the same assignee as the present invention and incorporated herein by reference, discloses a cell size of 18 pixels. A unit cell of 18 pixels can produce up to 19 different grey-scale levels for each color, from pure white (blank) to fully saturated (all pixels colored). The 18 pixels of a unit cell can be arranged in any shape desired, although it is generally desirable to minimize the radius of a circle which would completely enclose the cell. It is also necessary for the shape to be spaced-filling (i.e., no pixels of the image plane appear between a two adjacent unit cells). An example of such a unit cell is shown in FIG. 1.
However, because only 19 grey-scale levels are obtainable with an 18 pixel unit cell, the discrete shifts between the adjacent grey-scale levels are noticeable to the human eye, as shown in FIG. 2A. The problem of abrupt density shifts can be reduce by employing a larger halftone cell with more grey-scale level. However, this has the undesirable effect of making the halftone pattern more visible. Currently, it has been proposed to also reduce the halftone cell visibility by grouping a plurality of the unit cells into a large halftone cell. Commonly, four such unit cells are grouped into the large halftone cell, which is commonly called a "quad dot". In operation, the halftone screen is dispersed over the four unit cells of the quad dot, such that one pixel of each of the four unit cells of a quad dot is filled in before a second one of any of the unit cells of the quad dot are filled in. In this matter, the 19 levels of a single unit cell can be spread out and duplicated over 4 times the area, generating 76 (4.times.19) different grey-scale levels. Accordingly, as shown in FIG. 2B, the difference between any two adjacent grey-scale levels is indistinguishable to the human eye and the degree of saturation or the density of the image can vary continuously.
However, as is noticeable in FIG. 2B, a quad dot halftone scheme generates vertical lines in approximately one half of the grey-scale levels. These vertical lines are caused by the filled-in pixels within a quad dot aligning, and the aligned pixels within each quad dot aligning with the aligned pixels of its vertically adjacent neighbors. Therefore, while the problem of abrupt density shifts between grey-scale levels when using unit halftone cells is reduced, there is still a problem of artifact patterning or a "visible texture" generated in the image when using a quad dot halftone cell.